Make the db table for CascadeClipboard configurable

[jrief]

The database table cmsplugin_cascade_cascadeclipboard is rarely used, and if, only by the staff users, hence not critical to the operation.

It thus could be shared between the database running the staging and the production project. This would allow the staff users to prepare a plugin tree on the staging system, save it to the clipboard and use it on the production system, in order to set the content of existing pages.